error resilience: check error_concealment, not err_recognition.

err_recognition is supposed to trigger detecting and reporting errors,
not trying to fix them.
lavc: change all decoders to behave consistently with AV_EF_CRCCHECK.

Just crccheck prints a warning, crccheck+explode returns an error.

Also document this behavior.
lavc: disable CRC checking by default
avfilter: have avfilter_get_by_name return const for next bump

Signed-off-by: Anton Khirnov <anton@khirnov.net>
avfilter: fix const use of avfilter_next

Signed-off-by: Anton Khirnov <anton@khirnov.net>